Field of the Invention
The present invention relates to an image forming apparatus utilizing an electrophotographic process such as a copier, a printer, a facsimile machine, or a multifunctional apparatus implementing these functions.
Description of the Related Art
Conventionally, various image forming apparatuses utilizing an electrophotographic system or an electrostatic recording system have been developed. Among these, an image forming apparatus employing a tandem system is known as an apparatus capable of forming a full-color image quickly. The image forming apparatus employing a tandem system is configured such that process cartridges respectively configured to form images of different colors are arranged in a line and the images of respective colors are transferred onto an intermediate transfer belt through primary transfer. Various kinds of image information have come to be treated due to the development of an information society, and the image forming apparatus employing a tandem system is increasingly utilized not as an exclusive printing apparatus for full-color printing but as a general-purpose apparatus that can be also used for monochromatic printing.
Japanese Patent Laid-Open No. 2003-337454 proposes an image forming apparatus employing a tandem system configured to separate other photosensitive drums than a photosensitive drum for black toner from an intermediate transfer belt when performing printing by using only black toner.
Japanese Patent Laid-Open No. 2013-109378 proposes an image forming apparatus employing a tandem system configured to bring also other photosensitive drums than a photosensitive drum for black toner into contact with an intermediate transfer belt and apply a bias also to other primary transfer portions than a primary transfer portion for black toner when performing printing by using only black toner.
In the case of printing a black and white image by using an image forming apparatus employing a tandem system, separating the other photosensitive drums than the photosensitive drum for black toner from the intermediate transfer belt as proposed in Japanese Patent Laid-Open No. 2003-337454 has a merit that the photosensitive drums are prevented from wearing while being separated from the intermediate transfer belt. However, in this system, the intermediate transfer belt and the photosensitive drums are required to be brought into contact with or separated from each other each time printing of a full-color image and printing of a black and white image are switched, and thus the printing throughput is restricted by operation time of a contact/separation mechanism.
Therefore, in some cases, a black and white image is printed with the other photosensitive drums than the photosensitive drum for black toner in contact with the intermediate transfer belt even when performing printing by using only black toner. In addition, there is a configuration employing a direct current charging system, in other words, a DC charging system, as a system for electrifying a photosensitive drum. In the DC charging system, only a DC voltage is applied for charging. Charging performance and the degree of convergence of potential of the DC charging system are low compared with a system in which an alternate current voltage, in other words, an AC voltage, is superposed on a DC voltage, and thus a case where the DC voltage is applied to an image forming portion in which image formation is not performed can be considered. In this case, if a primary transfer bias is not applied between the photosensitive drum and the intermediate transfer belt, a charging potential of the photosensitive drum will be deviated from a desired potential. This may cause, for example, carrier attachment in which carrier attaches to a photosensitive drum.
There is a configuration in which the peripheral speed of an intermediate transfer belt is set to be faster than the peripheral speed of a photosensitive drum. In the case where a primary transfer bias is applied in this configuration, a larger load is put on a driving roller that drives the intermediate transfer belt due to an electrostatic attraction force between the intermediate transfer belt and the photosensitive drum. Further, in the case where a slip occurs between the driving roller and the intermediate transfer belt and thus the speed of the intermediate transfer belt becomes lower than the speed of the photosensitive drum, the intermediate transfer belt may be loosened in a region upstream of a secondary transfer nip portion and thus an abnormal image may be formed due to an abnormal electric discharge at the secondary transfer nip portion.